1. Introduction
Flow restrictors are used in many applications including in mass flow controllers. Conventional flow restrictor consists of sintered alloy metal or a filter, in which their flow resistance cannot be controlled in the manufacture process against the allowable design value range in performance. A designer often needs to select a suitable flow restrictor from off the shelf manufactured ones to obtain proper flow restrictors. Furthermore, the flow restrictors can show a non-linear characteristics in use of flow measurement.
The conventional flow restrictors are built of sintered alloy metal or mesh-like elements. These flow restrictors are used as a buffer in flow pressure for flow control equipment. For example, flow meters could take advantage of the differential pressure and flow characteristics of the flow restrictors.
For this type of flow restrictors made of sintered alloy material, it is difficult to control the degree of porous density. Further, such products must be selected from off-the shelf manufactured products to obtain a certain specific range of flow resistance.
2. Problems in Conventional Sitntered Flow Restrictors
Sintered elements are believed to be not as clean as restrictor designs in thermal mass flow controllers.
The performance of the sintered elements often deteriorates at flows above 5 SLM. The sintered element restrictors lose a significant portion of the desirable non-linearity as the flow through it takes on a sonic (rather than laminar) characteristic. This is because of the increased hydraulic diameters used by the higher flow through flow restrictors.
The sintered elements often do not operate at lower inlet pressures to the mass flow controllers. This would necessitate a higher inlet pressure, which is often a barrier to wide spread acceptance in the existing market place. A lower pressure drop is therefore desired.